Project Summary/Abstract Inhibitory circuits formed by GABAergic interneurons (INs) contribute to processing and encoding of cortical information by shaping the spatial and temporal structure of neural activity. Consistent with this critical role of INs in normal brain functions, IN malfunction has been implicated in a wide array of brain disorders such as schizophrenia, autism, and epilepsy. Despite their importance, detailed wiring diagrams of inhibitory local circuits remain largely unknown due to huge diversity of IN types. Furthermore, it is poorly understood what principles govern assembly of inhibitory microcircuits. Filling these knowledge gaps will provide us with wiring and developmental principles of cortical inhibitory circuits, which in turn dramatically facilitate our understanding of how cortical circuits work. One fundamental cortical circuit module contains an excitatory principal neuron (PN) locally innervated by distinct IN subtypes (an IN-PN circuit). In the neocortex, PNs are grouped by areas, layers, and remote projection targets, which represent their functional attributes. It has been shown that distinct classes of PNs display unique homotypic- and heterotypic-connections and convey different neuronal signals. However, little is known about cellular and axonal organization of distinct IN subtypes sending inputs to defined PNs. To address this question, we have developed a novel genetic strategy combining rabies virus (RV)-mediated retrograde monosynaptic labeling and intersectional approaches. The major objective of our proposal is to provide wiring and developmental principles of INs sending inputs to defined PN subtypes at a cell type-specific resolution. Previous studies showed that layer 5 (L5) PNs receive a larger number of inhibitory inputs from parvalbumin (PV)-expressing INs than L 2/3 PNs and this connection feature is controlled by PN identity. Thus, we hypothesize that distinct PN types defined by areas, layers, and long-range projection targets have different organization of input INs, which is shaped at least in part by PN identity. To test this hypothesis, we will dissect the following subjects using an intersectional retrograde monosynaptic tracing, genetic manipulation of PN identity, and mouse genetics. In Aim 1, we will elucidate organization of PV-, somatostatin (SOM)-, or vasoactive intestinal polypeptide (VIP)-expressing INs sending inputs to distinct PN types defined by cortical areas, laminar positions, and remote projection targets. In Aim 2, we will examine developmental processes of IN-PN circuits containing specific PN types innervated by PV-, SOM-, or VIP-INs. In Aim 3, we will generate ectopic PNs by genetic manipulations of transcription factors that control PN identities and examine organization of input IN subtypes. Through these experiments, we will gain wiring and developmental principles of IN-PN circuits in a cell type-specific manner, which will pioneer novel approaches for diagnosis and treatment of brain disorders.